It is known to provide individual motors for the driven wheels of an electrically powered vehicle. For example, dual motors are commonly applied to the two drive wheels of a three-wheeled vehicle of the type wherein the third wheel is used for steering. When considering the oppositely mounted drive wheels in a vehicle of this type, it is obvious that, during a turning maneuver, the geometry of the turning radius forces one wheel (i.e., the inside wheel) to cover less distance than the other wheel (i.e., the outside wheel). With these vehicles it is, therefore, necessary to change the speed of one motor relative to the other as a turn is being carried out. If the torque applied to both wheels continues to be the same, independent of the steering angle, the inside wheel tends to slip and wear against the opposing surface.
One common method of dealing with this problem heretofore has been to use an angle sensing switch on the steering wheel and to then switch off the power to the inboard motor once the steering angle has reached some preset value (e.g., a 15.degree. turning angle). Controlling the drive motor in this manner, however, produces a lack of continuity in the vehicle's operation and detracts from its "feel" in a turning maneuver. There is also a lack of maneuverability which is particularly noticeable when the vehicle is operated on a slippery surface such as a dock plate. To better deal with these problems, some efforts have been made to individually control the torque on each motor using separate power regulators which are controlled as a function of the sensed steering angle. However, although these separate power regulation methods are seen as an improvement, other and more effective means of torque proportioning control have been sought by workers in the field.
Another, more general, problem arising during a turning maneuver of an electric vehicle relates to the speed at which a turn can safely be made. It is, of course, common sense that a vehicle frequently needs to be slowed down when a turn is being made. It nevertheless happens that inexperienced operators, and others, will execute a turn at an excessive speed. In view of the potential safety problems arising out of this kind of operation, some means has also been sought to automatically slow a vehicle during a turn and to cause it to be increasingly slowed as the sharpness of the turn increases.
Accordingly, it is among the objects of the present invention to provide a control system for an electric vehicle, which overcomes those problems outlined above; a control system which continuously and individually adjusts the field excitation of the drive motors so that torque and speed of the separate wheels are apportioned to the wheels as a function of the steering angle of the vehicle.